Preheat oven to 375ยบ with rack in the center. In a large skillet, melt butter over medium-high heat. Add carrots, celery, onion, thyme, and 1/2 teaspoon salt, and cook, stirring often, until softened, 5 to 7 minutes. Add flour, and stir to coat vegetables. Cook for 2 minutes more, then add milk, broth, mustard, garlic powder, and remaining 1/2 teaspoon salt, whisking to combine. Increase heat to high and bring to a boil. Immediately reduce heat to medium and simmer, stirring often, until thickened, 3 to 5 minutes. Fold in chicken and peas.